1. Field of the Invention
This invention relates, generally, to erosion resistant coatings for various substates, such as steel (e.g. stainless steel) and titanium substrates, and more particularly to novel layered erosion-resistant coatings which may be applied to steel and titanium compressor components of gas turbine engines to provide erosion resistance without exhibiting a sharp drop in fatigue life of the substrate alloy after the coating is applied.
2. The Prior Art
Gas turbine engine compressor blades are conventionally fabricated from various steel and titanium alloys. These blades are typically subjected to severe erosion when operated in sand and dust environments. It is blade erosion that reduces compressor efficiency, requiring premature blade replacement thereby resulting in increased overall costs.
There are presently available a wide variety of various erosion resistant coatings taught in the prior art such as tungsten and carbon coatings (U.S. Pat. No. 4,147,820), platinum metal coatings (U.S. Pat. No. 3,309,292) and boron containing coatings (U.S. Pat. No.2,822,302). However, these and other known coatings, which have been identified by the art for imparting erosion resistance to metallic substrates, such as titanium and steel alloy compressor blades, promote sharp drops in fatigue properties of the substrates. This results in the initiation of cracks and fractures with an attendant reduction in the service life of the substrate. This effect on the fatigue life of the substrate is believed due to the fact that the erosion-resistant coatings taught by the prior art are hard materials which produce residual stress and accompanying strains in the substrate thereby accelerating a reduction in the fatigue strength of the substrate. Since this cannot be tolerated, there exists a need in the art to avoid this disadvantage and to produce erosion-resistant coating systems which do not deleteriously affect the fatigue life of the substrate to which they are applied.
There are other examples in the prior art of various attempts to coat metallic substrates similar to examples described above. They are as follows: U.S. Pat. No. 3,640,689 describes a method of chemical vapor deposition of a hard layer on a substrate. The method includes providing an intermediate layer of a refractory interface barrier, such as a refractory metal, between the substrate and hard coating to prevent deleterious interaction between the substrate and the hard metal layer and to obtain a hard wear surface. A 0.2 mil thickness of tungsten deposited at temperatures of about 1000.degree.-1200.degree. C. is given as an example of an intermediate layer, and several carbide materials (e.g. TiC, HFC, and ZrC), are disclosed as the hard metal outer coating for substrates such as cutting tools formed of a cobalt based alloy.
U.S. Pat. No. 3,814,625 describes the coating of certain substrate materials, such as tool steel, bearing steel, carbon or boron fibers with tungsten and/or molybdenum carbide, and in some cases the use of an interlayer of nickel or cobalt between the substrate and coating to provide better adhesion. The patent also describes that when depositing the carbide outer layer, amounts of free metallic tungsten and/or molybdenum can be co-deposited with their carbides, and that some coatings may contain 10% or less by weight of tungsten in elemental form.
U.S. Pat. No. 4,427,445 describes a procedure whereby hard deposits of an alloy of tungsten and carbon are deposited at relatively low deposition temperatures on metallic substrates, such as steel. The substrate can include an interlayer of nickel or copper between the substrate and carbide to protect the substrate from attack by the gases used to deposit the carbide hard coating.
Other similar prior art methods and products are described in U.S. Pat. Nos. 3,890,456, 4,040,870, 4,055,451, 4,147,820, 4,153,483 and 4,239,819.